Soil Improvement and Ground Modification

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UNIVERSITI MALAYSIA PAHANG Department of Civil
and Environmental Engineering

• Chapter 4-3
• Soil Improvement and Ground Modification

Muzamir bin Hasan, M.Eng. Lecturer
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Stone Columns

• A method now being used to increase the
  load-bearing capacity of shallow foundations on
  soft clay layers is the construction of stone
  columns.
• Constructions of a stone column generally consist
  of water jetting a vibroflot into soft clay layer
  to make a circular hole that extends through the
  clay to firmer soil.
• The hole is then filled with an imported gravel.
  The gravel used for the stone column has sizes
  ranging from 6-40mm.

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Stone Columns

• Stone columns usually have diameters of 0.5-0.75m
  and are spaced as about 1.5-3m center-to-center.
• After the construction of stone columns, a fill
  material should always be placed over the ground
  surface and compacted before construction of the
  foundation.
• The stone columns tend to reduce the settlement
  of foundations at allowable loads.

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Stone Columns

• As described above, stone columns are columns of
  gravel constructed in the ground. Stone columns
  can be constructed by the vibroflotation method.
• They can also be installed in other ways, for
  example, with help of a steel casing and a drop
  hammer as in the Franki Method. In this approach
  the steel casing is driven in to the soil and
  gravel is filled in from the top and tamped with
  a drop hammer as the steel casing is successively
  withdrawn.

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Sand Compaction Piles

• Sand compaction piles is a cost-effective method
  of ground improvement which is commonly used to
  improve soft seabed soils prior to land
  reclamation works.
• This method involves driving closely-spaced sand
  columns into the soft seabed to form a grid of
  sand columns, which imparts higher strength and
  stiffness to the improved ground.
• The installation of these sand compaction piles
  often involves a large amount of cavity expansion
  displacement of the soft clay around the sand
  piles, which in turn, leads to significant
  changes in the strength and stiffness of the soft
  clay around the sand piles.
• In practice and research, the properties of the
  soft clay are taken to be those of the in-situ
  soil. 

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Sand Compaction Piles

• Similar to stone columns, and they can be used in
  marginal sites to improve stability, control
  liquefaction, and reduce settlement of various
  structures.
• These piles can significantly accelerate the pore
  water pressure-dissipation process and hence the
  time for consolidation when built in soft clay.
• Sand compaction piles are constructed by driving
  a hollow mandrel with its bottom closed during
  driving.

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Sand Compaction Piles

• On partial withdrawal, the bottom doors open.
  Sand is poured from the top of mandrel is
  withdrawn.
• The piles are usually 0.46-0.76 m in diameter and
  are placed at about 1.5-3m center-to-center.
• The pattern of layout of sand compaction piles is
  the same as for stone columns.

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Sand Compaction Piles
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Compaction Piles

• Installing compaction piles is a very effective
  way of improving soil. Compaction piles are
  usually made of prestressed concrete or timber.
  Installation of compaction piles both densifies
  and reinforces the soil. The piles are generally
  installed in a grid pattern and are generally
  driven to depth of up to 60 ft.

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Compaction Grouting

• Compaction grouting is a technique whereby a
  slow-flowing water/sand/cement mix is injected
  under pressure into a granular soil. The grout
  forms a bulb that displaces and hence densifies,
  the surrounding soil. Compaction grouting is a
  good option if the foundation of an existing
  building requires improvement, since it is
  possible to inject the grout from the side or at
  an inclined angle to reach beneath the building

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Compaction Grouting
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Soil Stabilization

• Admixtures are occasionally used to stabilize
  soils in the field particularly fine-grained
  soils.
• The most common admixtures are lime, cement, and
  lime-fly ash.

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Soil Stabilization

• The main purposes of soil stabilization are to
• Modify the soil
• Expedite construction
• Improve the strength and durability of the soil

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Soil Stabilization

• When mechanical stabilization does not increase
  the reclaimed materials load carrying capability
  sufficiently, other additives can be used in
  conjunction with the granular materials or on
  their own. The addition of these additives is
  referred to as chemical stabilization. The more
  common additives are Portland cement, lime, and
  fly ash.

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Soil Stabilization

• Portland CementPortland cement has proven to be
  a very good method of base stabilization.
  Although there is no nationally accepted method
  for undertaking an mix design, it is generally
  recognized that a mix of Portland cement between
  2 to 6 of the dry aggregate weight produces the
  desired results.

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Soil Stabilization

• LimeLime is another common stabilizing additive
  but is used mainly for subgrades and soils with a
  high PI
• Fly AshFly ash is a by-product of coal-fired
  power production and can also be a very cost
  effective stabilization additive where it is
  available at a reasonable price. There are
  basically two types of ash product in coal-fired
  power plants a lightweight, fine ash called fly
  ash and a coarser ash called bottom ash.

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Lime Stabilization

• The types of lime commonly used for stabilization
  of fine grained soils are hydrated high-calcium
  lime Ca(OH)2, calcitic quick lime (CaO),
  monohydrated dolomitic lime Ca(OH)2.MgO and
  dolomitic quick lime.
• The quantity of lime used for stabilization of
  most soils usually is in the range of 5 10.

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Lime Stabilization

• When lime is added to clayey soils, several
  chemical reactions occur, cation exchange and
  flocculation agglomeration, and they are also
  pozzolanic.
• In the cation exchange and flocculation-agglomerat
  ion reactions, the monovalent cations generally
  associated with clays are replaced with divalent
  calcium ions.

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Lime Stabilization
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Lime Stabilization
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Lime Stabilization
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Lime Stabilization
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Lime Stabilization
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Lime Stabilization
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Cement Stabilization

• Cement is increasingly used as a stabilizing
  material for soil, particularly for the
  construction of highways and earth dams.
• Cement can be used to stabilize sandy and clayey
  soils.
• As in the case of lime, cement helps decrease the
  liquid limit and increase the plasticity index
  and workability of clayey soils.

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Cement Stabilization

• For clayey soils, cement stabilization is
  effective when the liquid limit is less than 45
  50 and the plasticity index is less than about
  25.
• Like lime, cement helps increase the strength of
  soils, and strength increases with curing time.
• Granular soils and clayey soils with low
  plasticity are most suitable for cement
  stabilization

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Cement Stabilization

• Calcium clays are more easily stabilized by the
  addition of cement, whereas sodium and hydrogen
  clays, which are expansive in nature, respond
  better to lime stabilization.
• For these reasons, proper care should be given in
  the selection of stabilizing material.

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Cement Stabilization

• For field compaction, the proper amount of cement
  can be mixed with soil either at the site or at a
  mixing plant and then carried to the site. The
  soil is compacted to the required unit weight
  with a predetermined amount of water.

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Cement Stabilization

• Similar to lime injection, cement slurry made of
  Portland cement and water (water-cement ratio
  0.55) can be used for pressure grouting of poor
  soils under foundations of buildings and other
  structure.
• Grouting decreases the hydraulic conductivity of
  soils and increases the strength and the
  load-bearing capacity.

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Cement Stabilization

• For design of low-frequency machine foundations
  subjected to vibrating forces, stiffening the
  foundation soil by grouting and thereby
  increasing the resonant frequency is something
  necessary.

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Cement Stabilization

• Portland cement is spread evenly over the surface
  of the existing asphalt in the amount of 2-4 of
  the total depth to be recycled and mixed in.

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Cement Stabilization

• Water is added usually by water truck or by hose
  if necessary.

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Cement Stabilization

• A grader then mixes the material and rebuilds the
  crown.

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Cement Stabilization

• The mixture is rolled with a roller or
  multi-wheel roller and left to cure for a couple
  of days. After curing, surface can be chip-sealed
  or paved.

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Fly Ash Stabilization

• Fly ash is a by product of the pulverized coal
  combustion process usually associated with
  electric power generating plants.
• It is a fine grained dust and is primarily
  composed of silica, alumina, and various oxides
  and alkali. It is pozzolanic in nature and can
  react with hydrated lime to produce cementitious
  products. For that reason, lime-fly ash mixtures
  can be used for stabilization of highway bases
  and subbases.

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Fly Ash Stabilization

• Effective mixes can be prepared with 10-35 fly
  ash and 2-10 lime. Soil-lime-fly ash mixes are
  compacted under controlled conditions with proper
  amounts of moisture to obtain stabilized soil
  layers.
• Its use may eliminate the need to add
  manufactured lime.

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Fly Ash Stabilization